Guava基础

什么是Guava？

Guava是Google开源的一个涵盖了字符串处理、缓存、并发库、事件总线、IO等常用操作的Java核心库，也是google自己很多java项目依赖的工具库。

总的来说，就是google对java api优化，自己实现的工具库。

核心方法

    //测试代码的公共部分
    @Before
    public void setUp() throws Exception {
        guavaDemo = new GuavaDemo();
    }

    @After
    public void tearDown() throws Exception {
        guavaDemo = null;
    }

Preconditions 对参数做前置检查,经常用在方法的最前面，对参数进行校验，不符合则抛出异常

checkArgument方法参数为空时，抛出java.lang.IllegalArgumentException

  /**
     * Guava参数无效检查
     * @param user
     */
    public void checkArg(User user){
        Preconditions.checkArgument( user != null, "user null error");
    }

@Test
    public void testCheckArg(){
        try {
            guavaDemo.checkArg(null);
            fail("user argments is error!");
        } catch (Exception e) {
            e.printStackTrace();
            assertTrue(true);
        }
    }

参数为空时，抛出java.lang.NullPointerException异常

/**
     * 参数非空检查
     * @param user
     */
    public void checkArgNotNull(User user){
        Preconditions.checkNotNull(user);
    }

Optional

or() 传入空时，返回自定义的默认值

/**
     * or() 传入空时，返回自定义的默认值
     * @param s
     * 参数为空时，赋默认值
     * @return
     */
    public String optionalMethod(String s){
        String result = null;
        Optional<String> optional = Optional.fromNullable(s);
        if( optional.isPresent() ){
            return optional.get();
        }

        return optional.or("default string");
    }

@Test
    public void testOptionalMethodNull(){
        String s = guavaDemo.optionalMethod(null);
        assertEquals("default string", s);
    }

    @Test
    public void testOptionalMethodNotNull(){
        String s = guavaDemo.optionalMethod("test");
        assertEquals("test", s);
    }

or() 参数为空时，赋默认值，默认值通过lambda表达式覆盖方法获取

 /**
     * or() 参数为空时，赋默认值，默认值通过lambda表达式覆盖方法获取
     * @param s
     * @return
     */
    public String optionalMethodSupplier(String s){
        String result = null;
        Optional<String> optional = Optional.fromNullable(s);
        if( optional.isPresent() ){
            return optional.get();
        }

        result = optional.or( ()-> {
                return "default string";
            }
        );
        return  result;
    }


@Test
    public void testOptionalMethodSupplierNull(){
        String s = guavaDemo.optionalMethod(null);
        assertEquals("default string", s);
    }

    @Test
    public void testOptionalMethodSupplierNotNull(){
        String s = guavaDemo.optionalMethod("test");
        assertEquals("test", s);
    }

orNull() 传入空时返回null

/**
     * orNull()传入空时返回null
     * @param s
     * @return
     */
    public String optionalMethodOrnull(String s){
        String result = "";
        Optional<String> optional = Optional.fromNullable(s);

        result = optional.orNull();
        return result;
    }

@Test
    public void testOptionalMethodOrnull(){
        String s = guavaDemo.optionalMethodOrnull(null);
        assertNull(s);
    }

    @Test
    public void testOrnullReturnNotNull(){
        String s = guavaDemo.optionalMethodOrnull("test");
        assertNotNull(s);
    }

ComparisonChain

/**
     * ComparisonChain 提供两个Object的链式比较
     * @param user
     * @param user2
     * @return
     */
    public boolean comparisonChain(User user, User user2){

        return ComparisonChain.start()
                .compare(user.getName(), user2.getName())
                .compare(user.getAge(), user2.getAge())
                .result() == 0 ? true : false;
    }

Objects、MoreObjects
Strings、Joiner、Splitter、CaseFormat

ImmuableList、Multiset、Multimap

        GuavaDemo guavaDemo = new GuavaDemo();
        System.out.println(guavaDemo.optionalMethodOrnull(null));

        //Objects 是对java Object的扩展
        User user = new User("allen",15);
        User user1 = new User("allen",15);

        System.out.println(Objects.equals(user,user1));
        System.out.println(Objects.hashCode(user));

        System.out.println(MoreObjects.toStringHelper(user).add("name","testName").toString());

        user = null;
        System.out.println(MoreObjects.firstNonNull(user, new User("allen",12)));

        //Strings,Joiner,Splitter,CaseFormat
        //输入为空时转换成""
        System.out.println(Strings.nullToEmpty(null).equals(""));
        //复制字符串
        System.out.println(Strings.repeat("abc",3));

        //拼接字符串
        Joiner joiner = Joiner.on(";").skipNulls();
        System.out.println(joiner.join("1",null,"2","3"));

        //分割字符串
        String splitStr = Splitter.on(';')
                .trimResults()
                .omitEmptyStrings()
                .split("1 ;2;3;4;;;").toString();
        System.out.println(splitStr);

        //格式转化
        String userName = CaseFormat.LOWER_UNDERSCORE.to(CaseFormat.LOWER_CAMEL, "user_name");
        System.out.printf("userName: %s \n", userName);

        //ImmutableList,Multiset,Multimap
        ImmutableList<String> list = ImmutableList.of("1","2","3");
        list.stream().forEach(System.out::print);

        System.out.println();

        Multiset<String> set = HashMultiset.create();
        set.add("1");
        set.add("1");
        set.stream().forEach(System.out::print);

        System.out.println();

        //ArrayListMultimap允许重复的key
        Multimap<String,String> map = ArrayListMultimap.create();
        map.put("test","1");
        map.put("test","2");
        System.out.println(map.get("test"));

输出结果

null
false
746292446
User{name=testName}
User{name='allen', age=12}
true
abcabcabc
1;2;3
[1, 2, 3, 4]
userName: userName 
123
11
[1, 2]